Semi-lumped bandpass filter

ABSTRACT

The inventive bandpass filter is suitable for use in a multi-layer ceramic (MLC) structure. Specifically, the filter is a semi-lumped type, formed by lumped capacitors and distributed transmission lines. Preferably, shorted transmission lines are used as equivalent inductors, which are connected with high value lumped capacitors to form resonators. Accordingly, the size of the resonators can be much smaller than 1/4 of the signal wavelength. The bandpass filter preferably has attenuation poles very close to lower passband edge, which results in a very sharp rejection near the stopband. The inventive filter may be utilized as a receiver-end filter in, e.g., duplexer applications or for miniaturized applications, such as portable communications.

FIELD OF THE INVENTION

This invention relates to electrical filters. More particularly, thisinvention relates to so-called semi-lumped, multi-layer and miniaturizedfilters, which are formed from, e.g., multi-layer ceramic technology.The invention is suitable for use in mobile communication instruments,such as portable telephones and cordless telephones.

BACKGROUND OF THE INVENTION

Electrical filters are utilized to transmit desired electrical signalsby selecting or rejecting one or more signal components, related tofrequency. Filters may generally be grouped as lowpass, highpass,bandpass and bandstop filters depending on their characteristics. Alowpass filter passes low frequency electrical signals, while rejectshigh frequency electrical signals. Conversely, a highpass filtersuppresses the frequency electrical signals, while passes high frequencysignals. A bandstop filter, however, passes all signals except thesignals having frequencies between two selected frequency points. Abandpass filter, however, passes electrical signals in a particularfrequency band between two frequency points (e.g., a high and lowfrequency point).

Depending on the implementation of the filter, the types of filters canalso be grouped into three types, i.e., lumped, distributed andsemi-lumped (which is constructed by lumped and distributed elements).The size of distributed elements, which is usually a section of thetransmission line, is determined by the wavelength associated to theoperational frequencies, such that lower frequencies translate intolarger distributed elements. Accordingly, at frequencies lower than 200Mhz, semi-lumped type or distributed type filters are impractical due tothe unacceptably large size of the distributed elements. However, in thehigher microwave frequency and millimeter-wave frequency regions,distributed type filters are commonly used due to their acceptable sizeand typically better performance than the lumped type filters. Lumpedtype filters are conventionally not suitable at frequencies higher thanseveral hundred MHz as they become too lossy and are susceptible to highparasitic effects. Although the problem of signal loss in lumpedelements can be improved in superconductor structures, such applicationsare limited.

At frequencies between several hundred MHz to several GHz, therelatively large size of distributed elements is not practical, e.g., inmobile communication instruments. The trends in designing mobilecommunication instruments are to reduce both physical size and powerconsumption. There are two typical ways to design a high performanceminiaturized filter. The first is utilizing the semi-lumpedconfiguration, mentioned above, and the second is utilizing a highdielectric constant structure. Semi-lumped type filters usually employchip capacitors, interdigital type capacitors and/ormetal-insulation-metal (MIM) capacitors, which are not as lossy aslumped inductors, and sections of distributed transmission lines whichare usually much shorter than 1/4 signal wavelength. Besides theminiaturization advantages of the semi-lumped configuration, as comparedto distributed filters, this type of filter also has the ability tocontrol the suppression of the periodic spurious signals thatdistributed type filters usually suffer.

Recently, high dielectric constant ceramic filters, such as coaxial ormono-block types, have become very popular due to their high performanceand small size. The high performance is due, in part, to the round orsmooth curves exhibited in the cross section of the transmission linesgenerate less conductor loss. The small size is due, in part, to thefact that the dielectric constant of ceramic is very high, resulting ina reduction of the signal wavelength. Since these kind of bandpassfilters usually use 1/4 wavelength transmissions as resonators or "J, K"inverters, they are not suitable to be implemented in low dielectricconstant structures having a frequency region between several hundredMHz to several Ghz.

In addition, there are several other disadvantages utilizing ceramicfilters. Due to their 3D profiles, each design requires a new moldingmodel which is usually very expensive. Further, in fabrication of thecoaxial type ceramic filter, different coaxial resonators are separatelysintered and coated, and the blocks are electrically individuallyconnected to each other, usually by soldering the connecting wires byhand. Further, the separate blocks must be fastened to some mountingsupport in a mechanically reliable way. The above indicates that themanufacture is difficult and costly. Although mono-block type ceramicfilters are improvements of the coaxial type ceramic filters, they haveessentially reached their limits with respect to miniaturization.

In addition, the low temperature cofirable ceramic (LTCC) technique hasbecome popular in RF applications. Multi-layer ceramic (MLC) integratedcircuits based on the LTCC technique have several advantages, such asthe capability of mass production, 3D high integration, as well ascomprising buried resistors, inductors and capacitors. Further, almostall the passive components can be fabricated in one manufactureprocessing. Except that a portion of the MLC circuit surface area isused for mounting active devices, almost all passive components can beburied into different ceramic layers to save real estate. Accordingly,the size of RF module can be reduced.

However, as stated with respect to MLC circuits modules, the use ofceramic type occupies some the surface area and requires additionalprocessing steps to mount the active devices on the surface. Such userequires a larger surface area and increases manufacture complexity.Accordingly, there is a need for filters suitable to be fabricated andimplemented in MLC structures that does not inhibit circuitminiaturization.

Recently, laminated planar bandpass filters have been developed for MLCstructures. These filters consist of planar resonators that are buriedin a high dielectric constant MLC structure, and can therefore begrouped as semi-lumped type filters. These bandpass filters use coupledtransmission line sections and lumped planar capacitor pads for thecoupling capacitor of the transmission line resonators. Such laminatedplanar filters may be treated as the variations of comb-line or steppeddigital elliptic filters. However, the use of coupled transmission linesis disadvanteous because in MLC structures having low dielectricconstants, the signal wavelength is relatively large. Utilizing coupledtransmission lines further increases miniaturization concerns. Inaddition, the flexibility in designing laminated planar bandpass filtersis inhibited by the control criteria of coupled transmission lines.

Accordingly, it is an object of the present invention to overcome thedeficiencies in the prior art.

SUMMARY OF THE INVENTION

These and other objects are achieved by the present invention. To solvethe abovedescribed shortcomings of the prior art, the present inventiondiscloses inventive bandpass filters that are suitable in MLC structure.Instead of using coupled transmission lines, as with laminated planarfilters, the present invention utilizes weak or non-coupling (non-EM)lines. The characteristics of the equivalent circuit have severaladvantages as follows:

1. The MLC structure of the present invention is relatively simplistic.The structures use pi-type capacitor networks that can be easilyimplemented in an MLC structure.

2. The control of the filters is straightforward by adjusting thelengths of the transmission line sections during filter fabrication.

3. The resonators which control the attenuation poles are the so-calledsemi-lumped type. Preferably, the resonators include large value lumpedcapacitors and grounded transmission lines. Accordingly, the length ofthe transmission lines can be much shorter than a 1/4 signal wavelengthto resonate, which in turn inherently reduces the filter size.

4. As stated, the shorted transmission lines are considered as weak ornon-coupling. The isolation of the EM coupling between the transmissionlines can be easily achieved by bending such lines or placing a groundedstrip therebetween. This property can reduce the large spacingrequirement between the transmission lines that is usually necessary toachieve sufficient isolation the lines.

5. This invention is suitable to be applied in low dielectric constantMLC structures due to its semi-lumped nature. The MLC integratedcircuits are typically made of low dielectric constant materials, sincea 50 Ω transmission line or high impedance line are difficult to utilizein high dielectric constant MLC structures. Accordingly, this inventioncan be integrated with RF active circuits in low dielectric constant MLCstructures to fabricate compact and inexpensive RF circuit modules.

6. The embodiments of the present invention are configured to ensurethat the bandpass filter attenuation pole is in close proximity to thepassband edge. Accordingly, fewer resonators are required. Also, since asharp cutoff is evident near the passband edge, this invention issuitable to be used as a receiver-end filter in duplexer applications.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description, given by way of example and notintended to limit the present invention solely thereto, will best beunderstood in conjunction with the accompanying drawings, where similarelements will be represented by the same reference symbol, in which:

FIG. 1 schematically illustrates the inventive semi-lumped filter inaccordance with an embodiment of the present invention;

FIG. 2 graphically illustrates the performance of the bandpass filter ofFIG. 1 in terms of db vs. GHz in accordance with the present invention;

FIG. 3 illustrates an MLC structure utilizing microstrip circuitryhaving a first strip design in accordance with an embodiment of thepresent invention;

FIG. 4 illustrates an MLC structure utilizing microstrip circuitryhaving a second strip design in accordance with an embodiment of thepresent invention;

FIG. 5 illustrates an MLC structure utilizing stripline circuitry havinga first strip design in accordance with an embodiment of the presentinvention;

FIG. 6 illustrates an MLC structure utilizing stripline circuitry havinga second strip design in accordance with an embodiment of the presentinvention;

FIGS. 7A and 7B illustrate a MIM pi-type capacitor networks inaccordance with the present invention;

FIG. 8 illustrates the technique to equal the potential of groupedconductor plates using vias according to the present invention;

FIG. 9 illustrates a semi-lumped filter having conductor strips directedin different directions in accordance with the present invention;

FIG. 10 illustrates a microstrip line in accordance with the presentinvention;

FIG. 11 illustrates a stripline in accordance with the presentinvention;

FIG. 12 illustrates a multi-layer microstrip line in accordance with thepresent invention;

FIG. 13 illustrates a multi-layer stripline in accordance with thepresent invention;

FIG. 14 schematically illustrates the tuning method of the bandpassfilter in accordance wit the present invention; and

FIG. 15 illustrates an example of transmission line isolation inaccordance with the present invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

FIG. 1 illustrates a preferred embodiment of the present invention. Theinventive semi-lumped bandpass filter 50 includes ground nodes 01, 02,03, 04, 05, 06, 07 and 08, arms 01-1, 02-2, 03-3, 04-4, 05-5, 06-6, 1-5,2-3 and 4-6 which represent the lumped capacitors (or known as pi-typecapacitor networks), and strips 7, 8, 9 and 10 which represent sectionsof transmission lines.

Each pi-type capacitor network and grounded transmission line forms aresonator (with the transmission line acting as an inductor). In FIG. 1,illustratively, the resonance of the resonator is controlled bycapacitor 1-5 and transmission line 9. Note that the grounded capacitors1-01 and 5-05 are grounded and illustratively have a small value.

FIG. 2 graphically illustrates the performance of filter 50. FIG. 2,graphs attenuation poles 24 that appear close to the lower passband edge28, resulting in very sharp cutoff of outband signals. Further,attenuation poles 24 and 26 appear in the stopband 22, which positionscan be easily tuned by varying the length of the shorted transmissionline sections 9, 10. The property of the filter can be used to tune thefrequency shift and the width change of the stopband with high rejectiondue to, e.g., a manufacture error during fabrication.

Referring back to FIG. 1, it should be noted that filter 50 may befabricated using, e.g., microstrip circuitry, stripline circuitry orboth in such MLC structure which will be described in greater detailbelow. In addition, note that the resonators which control theattenuation pole in FIG. 1 is semi-lumped type. A first resonator isformed by capacitors 1-5 and grounded transmission line 9, and a secondresonator is formed by capacitors 4-6 and grounded transmission line 10.Accordingly, the resonators can be treated as series LC resonators. Ifthe capacitor value is large, then the inductor value is small if theresonance is at a fixed frequency point, such that the transmission linelength is proportionally short.

Further, with respect to FIG. 2, note that the bandpass filterattenuation pole is in close proximity to the passband edge.Accordingly, fewer resonators are required. Also, since a sharp cutoffis evident near the passband edge, this embodiment is suitable to beused as, e.g, a receiver-end filter in duplexer applications.

FIGS. 3 to 7 illustrate different embodiments of MLC structures, shownas pi type capacitor networks. FIG. 3 schematically illustrates an MLCstructure utilizing microstrip circuitry. Specifically, two couplemicrostrip lines 11, 33 are placed on dielectric substrate 55 having abottom ground plane 66. Preferably, substrate 55 is ceramic; however,other materials may be utilized, as desired. If specific filterrequirements mandate that capacitors 1-5, 2-3 or 4-6 must be largevalues, the strips may be designed, as shown in FIG. 4. Two interdigitalstrips 22, 44 are placed on the dielectric substrate 77 having a bottomground plane 88. The interdigital arrangement of the conductor stripsincreases the capacitance value of pi-type capacitor networks 1-3, 2-4.

FIG. 5 schematically illustrates an MLC structure utilizing striplinecircuitry. The circuit of FIG. 5 is similar to the circuit of FIG. 3;however, a dielectric substrate 56 and a top ground plane 57 are addedon substrate 55. In addition, the circuit of FIG. 6 is similar to thecircuit of FIG. 4, however, a dielectric substrate 76 and top groundplane 78 are added on substrate 77.

In a multi-layer structure, an MIM pi-type capacitor network can beimplemented, as shown in FIG. 7A. Their structure comprises severalconductor plates grouped as 84 and 85. These conductor plates areinserted in different layers of the dielectric substrate 82 betweenground planes 81, 83. In a multi-layer microstrip circuit, shown in FIG.7B, the top ground plane 83 and the dielectric substrate 82, between thetop conductor plate of 84 and the top ground plane 83, can be removed.Note that with the circuit of FIGS. 7A and 7B, the number of theconductor plates of 84, 85 may vary. Further, the positions of the inputpoint and the output point of 84, 85 may vary, as well, and theirpositions can be located at any layer and in any direction.

A technique to equal the potential of the grouped conductor plates 84,85 is shown in FIG. 8. Vias 86 are used to connect these conductorplates. Note that the number of vias 86 can vary. Additionally, theinput direction and the output direction of the pi-type capacitornetworks of FIGS. 3 to 7 may also vary, as shown, e.g., in FIG. 9. InFIG. 9, note that the conductor strips 11, 33 can be directed indifferent directions depending on the desired filter layout.

With reference back to FIG. 1, transmission lines 7 or 8 are shown ingreater detail in FIGS. 10 and 11, as a conductor strip 503. In FIG. 10,conductor strip 503 is placed on a dielectric substrate 502 with aground plane 501. This is the so-called microstrip line. In FIG. 11, adielectric substrate 504 with a top grouped plane 505 is placed ondielectric substrate 502. This is the so-called stripline. The conductorstrips 503 may be bent or meandered to minimize the size.

In addition, the transmission lines 9 or 10 of FIG. 1 can also beimplemented, as in FIG. 10 and FIG. 11. Additionally, they can beimplemented by the multi-layer strip line technique, as shown in FIG.12. For example, two dielectric substrates 604, 605 are placed betweenthree ground planes 601, 602, 603. Conductor strips 606, 607 areconnected by vias 608 passing through a "no conductor" area 609. Theconductor strips in FIG. 12 can also be implemented in structures withmore than two layers. Another implementation of lines 9 or 10 is shownin FIG. 13. This configuration is very suitable for fine filter tuningafter fabrication. The structure of FIG. 13 includes ground planes 701,702, dielectric substrates 703 and 704, microstrip line 706, andstripline 707. The connection of lines 706, 707 may be made by vias 708passing through the no conductor area 709. As with the structure of FIG.12, more than two layers can be fabricated. The transmission linesections 9 and 10 can also be bent or meandered in the same layer. Aspreviously stated, the shorted transmission lines are considered as weakor non-coupling. The isolation of the EM coupling between thetransmission lines can be easily achieved by bending such lines orplacing a grounded strip therebetween. This property can reduce thelarge spacing requirement between the transmission lines that is usuallynecessary to achieve sufficient isolation the lines.

FIG. 14 illustrates the tuning method of the bandpass filter.Specifically the ground connections of transmission lines 9 and 10 areshown. Lines 9 and 10 are grounded either by using a ground planeconnection or by directly connecting the lines to a grounded conductorsidewall. The embodiment of FIG. 14 reduces signal loss that may occurwhen grounding by vias. The structure of FIG. 14 includes microstripline or stripline 401, and a conductor pad 402 that is grounded by vias403. The filter tuning can be made by using a laser trimmer to trim thearea 404 for increasing the length of the transmission line 401. Notethat the shape of the grounded conductor pad 402 can vary depending onthe desired filter layout.

Since the transmission lines 9 and 10 are considered weak ornon-coupling (non-EM), they should be isolated. Usually, the isolationcan be achieved by bending transmission lines 9 and 10. However, theisolation may also be made by placing a conductor strip 903 grounded byvias 904 between transmission lines 901, 902, as shown in FIG. 15.

This invention is suitable to be applied in low dielectric constant MLCstructures due to its semi-lumped nature. The MLC integrated circuitsare typically made of low dielectric constant materials, since a 50 Ωtransmission line or high impedance line are difficult to utilize inhigh dielectric constant MLC structures. Accordingly, this invention canbe integrated with RF active circuits in low dielectric constant MLCstructures to fabricate compact and inexpensive RF circuit modules.

Finally, the above-discussion is intended to be merely illustrative ofthe invention. Numerous alternative embodiments may be devised by thosehaving ordinary skill in the art without departing from the spirit andscope of the following claims.

I claim:
 1. A semi-lumped bandpass filter, comprising:a first pluralityof π-type capacitor network including a first capacitor coupled to afirst node and a second node, a second capacitor coupled to the firstnode and a first ground node, and a third capacitor coupled to thesecond node and a second ground node; a second plurality π-typecapacitor network including a fourth capacitor coupled to a third nodeand a fourth node, a fifth capacitor coupled to the third node and athird ground, and a sixth capacitor coupled to the fourth node and afourth ground node; a third plurality π-type capacitor network includinga seventh capacitor coupled to a fifth node and a sixth node, an eighthcapacitor coupled to the fifth node and a fifth ground node, and a ninthcapacitor coupled to the sixth node and sixth ground node; a firsttransmission line strip coupled to the first node and the third node; asecond transmission line strip coupled to the fourth node and the fifthnode; a third transmission line strip coupled to the second node and aseventh ground node; a fourth transmission line strip coupled to thesixth node and an eighth ground node; a input port and a output portcoupled to the first node and the fifth node; wherein said filterincludes at least two semi-lumped resonators connected in series witheach other, said resonators comprising at least one of said first,second, and third plurality π-type capacitor networks coupled to atleast one of said first, second, third and fourth transmission linestrips.
 2. The filter of claim 1, wherein said transmission line stripsare formed on a first dielectric substrate, wherein said transmissionline strips may be formed by one of microstrip or stripline circuitry.3. The filter of claim 2, wherein said transmission line strips areformed by said stripline circuitry, such that said strips are formedbetween said first dielectric substrate and a second dielectricsubstrate.
 4. The filter of claim 3, wherein the striplines areconfigured to form a multi-layer ceramic (MLC) device.
 5. The filter ofclaim 1, further comprising at least one conductor side plane coupled toat least one of said transmission line strips, wherein said side planebeing grounded by at least one via, such that said substantiallynon-coupling transmission lines are grounded.
 6. The filter of claim 5,wherein said at least one conductor sideplane comprises a plurality ofsideplanes, each grounded by said at least one via which connect saidplurality of sideplanes.
 7. The filter of claim 1, wherein said filterbeing comprised in a radio frequency (RF) module.
 8. A semi-lumpedbandpass filter, comprising:a first plurality of π-type capacitornetwork including first capacitor coupled to a first node and a secondnode, a second capacitor coupled to the first node and a first groundnode, and a third capacitor coupled to the second node and a secondground node; a second plurality π-type capacitor network including afourth capacitor coupled to a third node and a fourth node, a fifthcapacitor coupled to the third node and a third ground node, and a sixthcapacitor coupled to the fourth node and a fourth ground node; a seventhcapacitor coupled to the first node and the third node; a firsttransmission line strip coupled to the second node and a fifth groundnode; a second transmission line strip coupled to the fourth node and asixth ground node; a input port and a output port coupled to the firstnode and the third node; wherein said filter includes at least twosemi-lumped resonators connected in series with each other, saidresonators comprising at least one of said first, second, and thirdplurality π-type capacitor networks coupled to at least one of saidfirst, second, third and fourth transmission line strips.